1. Field of the Invention
The present invention is directed to a constant dc current supplying circuit, and more particular to such a circuit including an inverter of which output is rectified to provide an output dc current and in which a switching element is driven to repetitively interrupt an input dc voltage of the inverter at varying duty ratios in a feedback manner by monitoring the dc current so as to increase and decrease the output dc current to a constant level.
2. Description of the Prior Art
In prior art, dc current supplying circuits including an inverter and a rectifier which provides an output dc current from an inverter output to a load, a feedback control is generally utilized to sense the output dc current for controlling the inverter to supply substantially constant dc current to the load. The inverter includes a switching element or transistor which is controlled by a controller normally by a PWM controller to switch the inverter input voltage at differing duty ratios in order to increase and decrease the dc current in response to the level of the sensed dc current for maintaining the output dc current at a constant level. For feedback control of the output dc current, the prior circuit is designed to include a differential amplifier producing an output signal that is a function of the difference between the sensed dc current level and a threshold level. The output signal, which is an analog signal, is directly fed back to the PWM controller for controlling the switching element of the inverter so as to produce the output dc current of a constant level. In order to prevent overshooting of the output dc current at the start of the dc current supplying circuit, the prior circuit is configured to have a delay or soft-start circuit which inhibits the operation of the PWM controller or the inverter for a limited time period immediately after the start of the dc current supply circuit. The delay circuit includes a capacitor which is charged by the common input voltage to the inverter and is connected to the inverter controller or PWM controller such that it enables inverter controller to drive the inverter in a feedback manner only after the capacitor is charged up to a certain level, whereby delaying the operation of the inverter by the limited time required for charging the capacitor. Although such delay circuit relying on the capacitor is successful at the start of operating or energizing the dc current supply circuit, it will not operate properly or fail to prevent the overshooting when the dc current supply circuit is re-energized within a short time interval after deenergizing the circuit or when the load is reconnected to the supply circuit within a short time interval after being disconnected from the circuit. This is because that, within such short interval, the capacitor is kept in a charged condition such that the inverter controller is still enabled to or ready to control the inverter in a feedback manner in prompt response to the sensed output dc current. Because the output dc current is sensed to be of zero level at a very moment of reenergizing the circuit or reconnecting the battery, the feedback control will be over-responsive to such zero current level to drive the inverter in the direction of rapidly increasing the output dc current and therefore causing the overshooting. At this overshooting condition, the switching element of the inverter will suffer from an excessive stress and may be damaged eventually after receiving repeated stresses. Therefore, the overshooting should be avoided in the dc current supplying circuit for protecting the switching element of the inverter and for providing reliable feedback control over an extended period of use.